Serpentine or seven transmembrane receptors mediate signals for a wide variety of stimuli, including neurotransmitters, hormones, chemoattractants, odorants, and light [Dohlman et al., Ann. Rev. Biochem. , 60: 653-688 (1991); Probst et al., DNA and Cell Biol., 11: 1-20 (1992)]. These receptors share several common structural features, including an extracellular amino terminus, seven transmembrane spanning domains, and a cytoplasmic carboxy terminus with clustered serine and threonine residues. More than 100 members of this superfamily of receptors have been identified. These receptors are coupled to intracellular signal transduction pathways by heterotrimeric GTP-binding proteins (G proteins) [Linder et al., Sci. Am., 267: 56-65 (1992)].
Two G protein-coupled signal transduction mechanisms have been especially well characterized: the hormone responsive .beta..sub.2 -adrenergic receptor mediates catecholamine stimulation of adenyl cyclase [Dohlman et al., supra] and the light receptor rhodopsin mediates phototransduction in retinal rod cells [Khorana, J. Biol. Chem., 267: 1-4 (1992)]. Both receptors specifically interact with G proteins following activation by ligand. This receptor stimulation is tightly regulated such that interaction with ligand leads to a rapid and reversible loss of responsiveness of the receptor to subsequent stimulation. The process is termed homologous desensitization and is caused by phosphorylation of the receptor, usually on a cluster of serines and threonines present at the carboxy terminus [Lefkowitz, Cell, 74: 409-412 (1993)]. Such phosphorylation is mediated by specific protein kinases which recognize the ligand-occupied receptor [Palczewski et al., Trends Biochem. Sci., 16: 387-391 (1991)]. The 62 adrenergic receptor kinase (.beta.ARK1) and rhodopsin kinase (RK) have been shown to phosphorylate the activated forms of the .beta.-adrenergic receptor and rhodopsin, respectively. Both proteins have been purified and enzymatically characterized in reconstituted in vitro systems.
Additional lines of evidence suggest that other G protein-coupled receptors may be regulated by specific kinases. Receptor phosphorylation has been shown to be involved in desensitization in a variety of G protein-coupled systems ranging from mammalian cells [such as muscarinic cholinergic receptors, Kwatra et al., J. Biol. Chem., 262: 16314-16321 (1987)] to slime mold [the chemotactic cAMP receptor, Klein et al., J. Cell Biol., 100: 715-720 (1985)] and yeast [the mating factor .alpha. receptor, Reneke et al., Cell, 55: 221-234 (1988)]. In addition, the carboxy terminal domain of most G protein-coupled receptors contains potential phosphorylation sites which may represent catalytic targets for such kinases.
Lorenz et al., Proc. Natl. Acad. Sci., 88: 8715-8719 (1991) compares the deduced amino acid sequences of human .beta.ARK1 [Benovic et al., Science, 246: 235-240 (1989)] and bovine RK and suggests that the two molecules are structurally related. While in principle these two protein kinases could be responsible for the desensitization of the whole family of G protein-coupled receptors, recent identification of other structurally related protein kinases suggests that this is not the case. Sequences encoding three other mammalian G protein-coupled receptor kinases (GRKs) have recently been cloned, rat .beta.ARK2 [Benovic et al., J. Biol. Chem., 266: 14939-14946 (1991)], human IT-11 [Ambrose et al., Hum. Mol. Genet., 1: 697-703 (1992)] and human GRK5 [Kunapuli et al., Proc. Natl. Acad. Sci. USA, 90: 5588-5592 (1993)], as well as sequences encoding two Drosophila GRKs (GPRK-1 and GPRK-2) [Cassill et al., Proc. Natl. Acad. Sci. USA, 88: 11067-11070 (1991)]. The GRK family has recently been the subject of the review article Lefkowitz, supra. All of the GRKs share the highest structural homology in the centrally located catalytic domain of approximately 250 amino acids. The amino and carboxyl regions surrounding the catalytic domain are less homologous and may confer substrate specificity and subcellular localization, respectively. The GRKs are expressed in a tissue specific manner, which may further aid in the regulation of G protein-coupled signal transduction events.
G protein-coupled receptor kinases that are expressed in leukocytic cells and tissues are likely to aid the function and activities of G protein-coupled receptors expressed in these cells. Consequently, such kinases are likely to be important mediator molecules in the immune system. For example, receptors for a number of chemoattractants including fMetLeuPhe [Thomas et al., J. Biol. Chem., 265: 20061-20064 (1990)] and C5a [Gerard et al., Nature, 349: 614-617 (1991)] and for the chemokines IL-8 [Holmes et al., Science, 253: 1278-1280 (1991)], GRO [Murphy et al., Science, 253: 1280-1283 (1991)] and MIP 1.alpha. RANTES [Neote et al., Cell, 72: 415-425 (1993)] have recently been identified as members of the G protein-coupled receptor superfamily. This suggests that modulation of G protein-coupled receptor kinase activity may influence health and disease states of the immune system in acute and chronic inflammation.
There thus exists a need in the art to identify G protein-coupled receptor kinases that are expressed in cells and tissues of the immune system. Elucidation of the DNA and amino acid sequences encoding such a kinase would provide information and material to allow the development of novel agents that selectively modulate the activity of the protein kinase.